1. Field of the Invention
This invention relates generally to a mooring system for floating storage vessels, and more particularly to mooring lines for floating storage vessels which may be used simultaneously for the transfer of fluid product.
2. Description of the Prior Art
Floating storage vessels for petroleum products are normally moored in the sea by mooring lines anchored to the sea bed. The mooring lines are normally secured to a turret mounted in a moon pool or opening in the vessel so that the vessel may rotate or "weathervane" about the turret. Separate risers extend from subsea wells or manifolds to the storage vessel for the flow of product from the subsea wells to the storage vessel. The risers are flexible and are connected to corresponding piping in the turret which extends to a manifold. A swivel stack is provided on the vessel with a separate product line from the turret manifold extending to a swivel chamber of each swivel for product supply to storage holds in the vessel. As examples of turrets, reference is made to U.S. Pat. No. 4,698,038 dated Oct. 6, 1987 and 5,306,186 dated Apr. 26, 1994, which are incorporated herein as a written description for all purposes. In regard to swivel stacks, reference is made to U.S. Pat. No. 4,306,741 dated Dec. 22, 1981 and 4,647,077 dated Mar. 3, 1987, which are incorporated herein as a written description for all purposes.
It is becoming commonly accepted practice for floating vessels (tankers, barges, column stabilized units, etc.) to be moored in the open ocean using spread or single point type mooring systems. This is being done using various combinations of chain, wire rope and polyester rope in conjunction with subsurface support buoys to provide mooring systems that are uniquely customized for a specific vessel and environmental conditions at a given site. In some cases, dynamic positioning using controlled thrusters may be used to augment the mooring system. Anchoring to the sea floor is normally done by use of drag imbedment anchors or anchor piles depending upon the sea bed soil conditions.
Such moored vessels are then outfitted with the mechanical equipment to receive and/or send the full range of petroleum related products from the sea floor to the vessel. The vessel and its equipment is then used for storage, processing and off loading by either transfer to other floating vessels or back to flowlines on the sea floor. For the diverse offshore oilfields located around the world, there tends to be a large number of flowline risers required between the turret and the sea floor. Systems are currently being manufactured and installed that have four (4), eight (8) or even as many as thirty (30) flowline risers.